Aqueous adhesive natural product ingredients are used for blending in certain aqueous adhesive formulae. For example, animal protein, casein, and starch have been used in a variety of applications including as envelope gums, corrugating board adhesives, label adhesives, and other applications. One class of adhesives using a natural product is the casein-based adhesive that is currently a leading class of labeling adhesive and is also used in other end uses. Casein adhesives possess a spectrum of beneficial properties useful in labeling applications including machinability, bond strength, and water resistance. Of these properties, water resistance can be very beneficial. Casein based adhesives can resist the action of water for 24 or more hours.
Another class of adhesive is the starch-based adhesive. One primary end use for starch-based adhesives discussed above is in the application of labels to beverage containers. In applying labels to beverage containers, high speed machines are used which transfer adhesive films while wet to pallets which then pick labels from label stacks and then transfer the adhesive to the label. Once coated with adhesive the label is contacted with the container for permanent adhesion. In the labeling process, when labels are adhesive coated and transferred from a stack to a bottle, many adhesive formulations tend not to have sufficient adhesive strength to adhere the adhesive film to the pallet or to the bottle during the transfer processes. Also certain adhesive compositions lack a property called "shortness" which relates to the resistance of the adhesive to forming long string-like portions of adhesive that can be created as the labeling machine transfers the wet label from place to place in the machine. Such "cobweb" or "angel hair" material while wasteful can also reduce the quality of the adhesive bond and can cause significant machine and processing difficulties because of the introduction of the angel hair into the machine.
Beverage bottles have been labeled with starch based adhesives for many years. However the use of starch and its derivatives in label adhesives can pose a significant problem when returnable beverage bottles are cleaned prior to refilling. Typically bottlers contact returned bottles with dilute hot caustic solutions to remove labels, disinfect and clean the bottles. Labels bonded with starch-based adhesives can resist removal under these conditions and can cause significant increases in costs of using returnable bottles. Casein-based adhesives are typically adequately cleaned by aqueous caustic.
One attempt in making a water resistant but alkali removable casein substitute in a starch-based adhesive is shown in Sirrota, U.S. Pat. No. 3,939,108 which teaches an adhesive that is combined with a styrene-maleic anhydride copolymer. Such an adhesive appears to have some utility in labeling beverage bottles, but has not achieved significant commercial impact to date.
As is the case with many natural product ingredients, the supply and pricing of casein has the potential for causing disruption in the availability of water resistant labeling adhesives. Casein has a variety of end uses and depending on demand casein can be in very short supply and can be available only at increased prices. A non-casein substitute labeling adhesive having the properties of casein-based aqueous dispersions that is formed from available natural and synthetic ingredients is a desirable goal.
Accordingly, a substantial need exists for developing new adhesive formulations and to develop a replacement formulation for casein-based adhesives. Further a need exists to improve starch-based adhesives that generally lend themselves to rapid and effective aqueous cleanability and particularly to selective aqueous or aqueous alkali cleanability.